1. Field of the Invention
The invention relates to a method of eliminating interference signals from video signals by means of motion-adaptive filtering with a second or higher-order transversal filter which is controlled by a motion detector.
2. Description of the Related Art
A method of eliminating interference signals from video signals by means of motion-adaptive filtering with a second-order transversal filter is known from DE 43 26 390 corresponding to U.S. Pat No. 5,519,453. In this method, video signals are derived from three consecutive pictures and, after evaluation with coefficients, added together. The coefficients are generated by a forward and a backward-motion detector. When there is no motion or only very little motion--the distinctions between the derived video signals are smaller than a given predetermined threshold value--, a maximum improvement of the signal-to-noise ratio of 4.8 dB can be achieved. In this case, the output signal is the arithmetic mean value of the three pictures. In the case of considerable motion, large amplitude differences between the three pictures generally occur. To avoid "smear" of the different motion phases, there should be a fading to the reference picture, i.e., the middle picture. To this end, the coefficients of the other pictures are controlled towards 0 and the coefficient of the reference picture is controlled towards 1.
A possible circuit arrangement for a motion detector is described in DE 43 19 342 A1, corresponding to U.S. Pat. No. 5,543,858. It comprises a forward-motion detector which, on the basis of the reference picture, evaluates the difference with the subsequent picture. An identically built backward-motion detector, which evaluates the difference with the preceding picture in a corresponding way, can then be added for a multidimensional motion detection. In this case, the amounts of the differences of two consecutive pictures are formed and submitted to a low-pass filtering operation so as to suppress high-frequency noise components. The low-pass filters may be implemented as two-dimensional (horizontal and vertical) transversal or median filters. To avoid response of the detectors at only a slight motion or at residual noise components, the signals are applied in accordance with an adjustable threshold characteristic. Subsequently, the coefficients are generated in a programmable read-only memory (PROM) in accordance with a declining characteristic. This characteristic starts at 1/3 for the static case (no motion or only little motion) and then decreases to zero with, for example, an exponential characteristic.
When the motion detectors were tested on improving the elimination of interference signals from video signals, it was found that a maximum interference signal reduction is achieved when the two motion detectors do not respond, i.e., at stationary pictures at which the S/N ratio improvement is 4.8 dB. An average interference signal reduction is achieved when only one of the motion detectors responds, with an S/N ratio improvement of 2.5 dB being achieved by evaluating the video signals with 2/3 for the reference picture signal and 1/3 for the signal of the stationary picture. If both motion detectors respond, no interference signal reduction is possible, i.e., the S/N ratio gain is 0 dB. This makes it clear that the original motion detector actually only allows S/N ratio improvement in a few cases only. An optimum filtering is not even reached once for the simple case where only one motion detector responds, because an optimum filtering operation with coefficients of 1/2 each for the reference picture signal and the signal of the stationary picture should then be used. This means that both pictures, between which there is no motion, should be averaged by 50% each, which would lead to an S/N ratio improvement of 3 dB. The reason for this detrimental behavior is that the mode of operation of one motion detector does not have any effect on that of the other.